I hope you enjoyed last week's world wind tour of physics.
But, I bet that you're ready to start thinking about astronomy again.
We're going to do that now, we're going to take all tools we acquired and
start applying them to understanding the rest of the universe.
It seems fitting to start with our local neighborhood, the solar system.
This is both, because our local neighborhood is very important.
It's where we live. And also because in a way, we know a lot
more about the solar system than about anywhere in this, rest of the universe,
because over the past few decades we've been able to send spacecraft robotic
spacecraft to visit many of the objects in the solar system.
Allowing us to verify the predictions we had made based on ground base
observations that are understanding of physics and compare them to actual
measurements. So we will be able to verify a lot of our
understanding of what goes on in the sky directly.
For both of those reasons, we're going to spend the next week studying the solar
system. I should point out at the beginning that
there are at least two aspects of the science of the solar system, that I will
not be able to give the appropriate time to.
This includes A, the amazing and fascinating history of space travel, in
our exploration of the solar system. that will be given short thrift because
we don't have time to put into it. And the other aspect is the intriguing
and exciting question of, the possibility for the existing of life elsewhere.
in the solar system or beyond and this is not something about which I know very
much and it's not the center of focus of this class.
So we will be discussing the physics of the solar system leaving the biology
aside. So with that said, what is the solar
system? Well, primarily the solar system is all
of the objects in bound orbits around the Sun and most of that is a star, it's the
sun To a great extent, 99.9% of the mass of the solar system is in this
average-sized, main sequence star we call our sun.
And this star dominates the solar system in more ways than containing all of the
mass. In addition to dominating the mass of the
solar system, the sun is extends its influence throughout the solar system in
two other ways. One, we talked about last week.
Is the radiation. The black body radiation with which the
sun glows. that is the source of our light and heat
here on earth and elsewhere in the solar system.
In addition, as we'll see as part of the processes that go in a star, the outer
atmosphere of the sun is a very, active place, and it is constantly emitting a
stream of charged particles called the solar wind that leave at high, the sun at
high speed and propagate throughout the solar system.
We will discuss this when we talk about the sun next week.
But this is a nontrivial flux of high-energy particles away from the sun.
And as we'll see it, it, it has lots of effects in the solar system.
To give you a sense, every 150,000,000 years the sun loses a full earth mass to
this solar wind. So there is this constant stream of stuff
coming out of the sun. Okay, we know about the sun.
We'll discuss the sun next week when we talk about the stars.
What else is out there? Well primarily what we think of is the
solar system, is the eight planets. The sun is orbited by eight large, round
objects, in slightly eccentric but almost exactly circular orbits of various sizes,
ranging from point four astronomical units. Mercury is the closest plane to
the sun, all the way out to 30 astronomical units, or two orders of
magnitude, of orbital radius all the way up to Neptune.
And we see in the this nice image that there's a nice cluster of very small
inner solar system orbits and then the more
widely spaced outer solar system orbits and essentially the space between the
planets is mostly empty. So, roughly, it's the sun with a planets
orbiting it and the orbits are distinguished not only by being circular
but note in this image, they are almost all in a plane.
So there are no planets that orbit the sun this way or that orbit the sun that
way. They all orbit in the same sense, in the
same plane. Clearly crying for explanation.
When one observes the sun as Galileo did when notices that the sun in fact spins
about an axis. And the sun's axis, surprise is
perpendicular to the ecliptic. So the sun, too, rotates about its axis
in the same sense as do the planets. the planets themselves come in two very
distinct families. There are the inner planets, Mercury,
Venus, Earth and Mars, the outer planets, Jupiter, Saturn, Uranus and Neptune and
they're different in as many ways as you can think of.
the inner planets are dense, as we shall see, as dense as rock or denser.
They are small, this image is not to scale, the left-hand side is to scale, so
that you see that Mars and Mercury are smaller than Earth and Venus.
the right hand side is the scale, which shows you that Jupiter is the largest.
But, between the right and left hand side is about a factor of ten.
In size, Jupiter in fact is eleven times as large in radius as Earth,
if we put the two figures to scale, and either we wouldn't see the inner planets
or the outer planets wouldn't fit. So, you have to draw a very sharp line
between the right and the left. The outer planets are not just bigger,
they are also far less dense. and their composition is different,
as we will see, the inner plants are rocky.
Their composition is silicates and iron and nickel.
The outer planets are mostly hydrogen, helium and Isis and lighter elements,
larger planets. Why this dichotomy?
Certainly an interesting question. Okay, we have eight planets.
The solar system is not just the sun and eight planets, there's other things
floating out there. What else is there?
Well, around many of the planets are objects that are captured in
gravitational Keplerian orbits about the planets as they orbit the sun.
They're called moons, we know of our moon.
There are a total of about a 150 Moons in the solar system most of them orbiting
Jupiter and Saturn, but all of the planets except Mercury and Venus have
moons. the outer planets tend to have many more
moons than the inner planets and some of these moons are planet sized.
Our moon is about almost the size of Mercury, some of the moons of Jupiter are
very large. Again notice that all of these large
objects, like the planets. All have the same shape.
So, they're all spherical. We want to understand why is everything
round. and where do these moons come from and
why do they orbit the planets and not the sun.
in addition to moons orbiting the planets, there are these beautiful
structures of rings. It turns out that, as we found in the
twentieth century, all of the giant planets have ring systems surrounding
them and we'll see that that's a ubiquitous fact.
But Saturn's rings are certainly a spectacular exception.
there's a reason why Galileo discovered the rings of Saturn and it was not until
the twentieth century that we found the other giant planets had rings.
and we'll want to understand what this structure is, where it came from, and
what it tells us. Looking at the moons and the planets of
the solar system we see another property that they all share, which tells us.
Something about what goes on in the solar system.
And that is, that they all seem to have suffered a little bit of damage.
this image is an image of Mercury, and you see that it looks kind of like the
moon. Because like the moon, it's pock-marked
with craters, the results of collisions some time in its past.
This image shows you that the Earth is not immune to such attacks.
This famous crater in Arizona was formed by an impact only 50,000 years ago.
And so, there have been and are impacts onto the Earth and this feature of
cratering is one of the most ubiquitous things we see.
Here's our familiar moon, with this is the nearside of the moon and the familiar
cratered shape of Tycho and so on shows us that the moon too has taken the brunt
of many hits. Here is Ganymede, Jupiter's moon and
Ganymede too has had the impacts. What caused this, is it still happening,
are we about to get hit? All questions we'll want to understand.
So we have planets, moons, rings, something that crashes into things and
creates craters. What else is out there?
Well turns out that's all the big stuff but there are smaller objects out there
The main collection of small objects is called asteroids.
this is a picture of a particularly pretty one, this is the asteroid Eros
close-up shot. And we see two things about Eros.
One is that, like everything else in the solar system, it has suffered some
collisions, it's cratered. But Eros is not round, it's shaped like a
potato. And in general asteroids,
do not necessarily follow the pattern we've seen where everything is spherical.
We'll have to understand why it is that things can come in various shapes.
There's this relatively densely populated region of the solar system.
There's about a tenth of an Earth mass orbiting in the asteroid belt and in
addition, for example in Jupiter's orbit, there are these so-called two groups of
so-called Trojan asteroids that orbit in Jupiter's orbit but 60 degrees east or
west of the planet and they orbit because,
It's a Kepler orbit with the same period as Jupiter.
Another thing we'll have to pay attention to.
The solar system, we think, can not possible end at Neptune.
There are objects that are a bit beyond Neptune's orbit, and our best hint about
them comes first from comets. Here's a pretty view of Haley's comet
from it's 1910 apparition. And it turns out that comets in their,
cometary orbits, where they approach the sun, cannot survive billions of years.
We'll talk about that later. And in order to explain the frequency
with which comets are observed entering the inner solar system in the plane of
the ecliptic. we conjecture the existence of a
collection of objects. Orbiting beyond Neptune's orbit.
It's called the Kuiper Belt, a disc-shaped distribution with it is
assumed tens of thousands of objects, hundreds of which are the size of the
moon or larger. Pluto would be an example of one of these
Kuiper Belt object. And.
these are objects which when perturbed from their orbits, form the source of the
short and intermediate range comets that we see orbiting in the plane of the
ecliptic and it turns out that in order to explain the frequency with which we
see comets, we can predict the density of objects that must be out in the Kuiper
belt. the Kuiper belt might extend out to as
much as 100 astronomical units away from the sun.
much farther away, it is conjectured that way past the orbits of the planets, is
this spherically symmetric halo of object.
the Oort cloud, it might have an inner Oort loud along the disk or the Hill
cloud but these objects orbit the sun in a highly eccentric,
Orbits outside the plane of the ecliptic, and they form the source for out long
period comets. This is our collection.
the first question you want to ask is What's it made of? And we can answer that
rather directly. the answer of course to, "What is the
solar system made of?" is mostly the answer of the question, "What is the Sun
made of?" because 99.9% of the mass is the Sun.
Of the.1 percent that is not the sun, 90% of the rest is Jupiter.
and Saturn and we have here a plot of the abundances of various atomic species in
the solar system, which roughly mirrors the abundance of the same atomic species
throughout the universe. So when we see what the solar system's
made of, we're really answering what the universe is made of.
We'll come back and try to understand this later in the class but for now it's
worth the observation and these are the relative numerical abundances of various
species of atoms, organized by atomic number.
And what you need, the, the scale is kind of random, but it's a logarithmic scale.
So that the fact that helium registers one less than hydrogen means that there's
about one helium atom for each ten hydrogen atoms.
There's a factor of ten less helium than hydrogen and together, because everything
is down by a factor of 10,000, they comprise almost the entire content of the
solar system by mass, because a helium atom weighs
four times as much as a hydrogen atom. that works out to about 70% hydrogen, 28%
helium and 2% the rest. everything else is trace amounts, down by
a factor, as I said, of at least 10,000, from the abundance of hydrogen and
helium. And this is a sobering note when you
remember that we and essentially, our entire planet is made up mostly of these
trace left-over materials. Astronomers tend to call anything that
is, heavier than lithium. So anything past the first three or four
elements. Metals, carbon is not a metal in the
chemical sense of the word. But in astronomy, heavier elements are
all called metals. So we say that the mass of the solar
system and the mass of the universe, essentially, has 2% metals.
But mostly, it's a world of hydrogen and helium.
That's our quick survey. this raises many questions.
I've listed some of them here and we'll try to see how many of them we can answer
and how many more we can raise as we get into more detail
Why are all the planets orbits circular? Why are they all in a plane?
Why are the comet orbits neither circular nor in a plane?
Why are all the planets and the large moons spherical objects and if so, why
are the asteroids not? Why are there two kinds of different
planets? Why are there inner planets of one kind
and outer planets of a completely different species of objects?
why aren't the asteroid is a planet? There's clearly stuff there.
there's room between Mars and Jupiter, why didn't a planet form there?
Why didn't asteroid belts form elsewhere? Why didn't the rest of the solar system
coalesced into planets? It's not a science question, but it's
always asked. What happened to Pluto to get it demoted
from planet to dwarf planet? What's a dwarf planet anyway?
what are rings? Why does Saturn come equipped with these
brilliant special rings? What made all the craters?
Where did it go, is it coming back? Are we about to get
it hit with some big impact. Remember we did our homework last week
and we saw that this can be a significant event.
Why do comets sometimes leave their comfortable orbits out in the Kuiper belt
and the Oort cloud and come diving into the solar system?
Why do some asteroids move from the asteroid belt into orbits that bring them
very near earth? We keep hearing of near-earth asteroids
and their potential for collisions. And,
if orbits can change, if something can move from a comfortable orbit out of the
Oort cloud and come crashing into the solar system.
Will the planets orbits change? Will the solar system remain in its
current configuration? Have planetary orbits changed?
Was the solar system always the way it is now?
And, underlying all of this, the question of
where did all of this come from, and when did that happen?
And we'll find that starting from the end, in fact, is the way we're going to
go in the next clip, we're going to figure out when it started and from that,
we'll try to go on to how it started. And that will try to generate as many
answers as we can to all of these questions.